When a fluid passes through a valve, it gains speed in the area where the cross section is reduced. Since the energy, according to Bernouilli's law, is conserved, the rise in kinetic energy generates a drop in the pressure energy. The latter then rises again when the passage cross section rises again, thereby reducing the speed of the fluid.
Hence, in standard regulation valves, the pressure can locally drop by a large amount and reach values that are smaller than the vaporization pressure. When this happens, water can vaporize and small gas bubbles may build up. The rise in pressure or the contact with a surface then leads to an implosion of these bubbles, which is a highly energetic phenomenon in terms of pressure and temperature which can lead to the surrounding solid matter being torn apart. According to the extent of the cavitation phenomenon, the erosion taking place on the downstream part of the valve body can be very quick and the destruction of the apparatus can take only a few weeks.
The implosion of the gas bubbles also generates a particularly large noise, which can be a problem for the neighbourhood if the apparatus is installed in an urban area or close to homes.
Consequently, when a valve is intended to be used in an application characterized by a large pressure difference, it is necessary to provide a regulation valve that is adapted to withstand such a large pressure difference.
Some technical solutions existing on the market consist in letting the fluid flow through planar or cylindrical surfaces, which may be provided with circular openings or slots. Some of these devices are designed so as to obtain several energy dissipation stages with the intention to distribute the pressure drop within the apparatus over the various stages to thereby reduce the appearance of the phenomenon of cavitation and the associated noise disturbances.
Such an anti-cavitation valve assembly is known from the published European patent application EP 1 794 483. The disclosed valve assembly for reducing cavitation includes a seat disposed within a valve housing intermediate a fluid inlet and outlet thereof. A disc guide is associated with the seat so as to be slidably movable relative to the seat. The seat includes a wall defining an inner chamber and having elongated slots formed therein so as to direct fluid towards the central portion of the fluid chamber. The disc guide includes a wall having elongated slots formed on an upper portion thereof and a non-slotted lower portion configured to substantially occlude the elongated slots of the seat when the disc guide is moved into a closed position. The seat furthermore includes an upper non-slotted wall portion to occlude the slots of the disc guide in the closed position. The device disclosed in the above mentioned published European patent application is constructed in such a way that two successive energy dissipations take place. A first pressure drop occurs when the fluid from the fluid inlet enters the seat through the elongated slots of the seat. Cavitation which may occur within the seat usually cannot harm the valve housing, since it is contained within the seat. The seat, however, can be configured as a spare part and easily exchanged. A second pressure drop occurs when the fluid leaves the seat through the elongated slots of the upper part of the disc guide towards the fluid outlet. Cavitation which may occur in this area may seriously and quickly damage the valve housing and lead to its complete destruction.
It appears, however, that this known valve assembly cannot guarantee the distribution of load loss over the various stages over the whole opening range of the valve assembly. For values of the valve opening that are smaller than 50%, which represents a typical range of this type of devices, most of the pressure drop takes place at the output of the disc guide. As a consequence, cavitation can occur and this in a critical area, since cavitation will occur in an area where the valve housing can be damaged. Hence, the prior art valve assembly is not efficiently protected against the cavitation phenomenon at small valve openings and the service life of the valve assembly is, therefore, adversely affected.
Another problem of the valve assembly disclosed in the above mentioned published European patent application is that the parts of the valve assembly have a rather complex shape. Indeed, the elongated slots of the disc guide are formed at an offset angle other than 90° with respect to the disc guide wall, in order to direct the fluid passing through the elongated slots towards the housing of the valve assembly at a non-direct angle. These non-radially oriented slots prevent the fluid stream from directly hitting the walls of the valve assembly upon leaving the disc guide. This specific orientation of the elongated slots renders the method of manufacturing the parts considerably more complex, which leads to higher manufacturing costs. Further, it renders the method of assembling the valve assembly more complex, as it requires an angular indexation of the disc guide within the seat. An imprecise mounting of the disc guide within the seat may also have consequences on the performance of the system.